Two important factors underlying the threshold visibility of sinusoidal gratings are the spatial inhomogeneity of the visual system and the local variation of receptive field sizes at each eccentricity. Psychophysical procedures are described which will permit the independent measurement of receptive field sizes (line spread functions) for S (sustained) and T (transient) mechanisms at several different eccentricities. Once the spatial properties of these mechanisms have been ascertained, the temporal impulse response functions will be independently measured. The data thus gathered on the spatiotemporal properties of S and T mechanisms will be applied to the prediction of spatial grating thresholds for various forms of temporal modulation. Two extensions of these studies are then planned. The first is to study the properties of S and T mechanisms under suprathreshold conditions. The second is to utilize grating stimuli to study hysteresis effects in stereopsis.